1. Field of the Invention
This invention relates to etchant baths for dissolution of metals and is more particularly concerned with improved methods and compositions for the etching of copper and copper alloys and, in a particular embodiment, with application thereof to the production of printed circuit boards.
2. Description of the Prior Art
The manufacture of printed circuit boards generally begins with a non-conducting substrate such as a phenolic or epoxy-glass sheet to one or both sides of which is laminated a layer of copper foil. A circuit is made by applying an etch resist image in the shape of the desired circuit pattern to the copper foil and subjecting the latter to the action of an etchant bath to etch away all the copper except that covered by the etch resist.
In the processes commonly employed in the art, the copper clad insulating board bearing the etch resist pattern is contacted either by immersion or by spraying with an acidic ferric chloride, cupric chloride or hydrogen peroxidesulfuric acid etchant or an alkaline ammoniacal etching solution. The etchants attack the copper where the metal surface is not protected by the resist. As the etching proceeds the resist-covered copper circuit pattern stands out in vertical relief. As the depth of the etch increases the sides of the copper supporting the resist are exposed to the etching solution and can be undercut resulting in circuit lines which do not have the designed cross-sectional area. This can cause problems in boards where impedance is tightly controlled.
Cupric chloride alkaline ammoniacal etchants are the ones most widely used commercially because of the high etch rates which they provide. A major drawback of this type of etchant is that the waste therefrom is difficult and expensive to treat and, since most etchant baths are operated on a feed and bleed type system, large volumes of such waste are generated. Electrolytic attempts to recycle or regenerate such baths have been largely unsuccessful due to the corrosive nature of the material and the large amounts of chlorine gas which are generated.
Cupric sulfate alkaline ammoniacal etchants do not pose such waste treatment problems and are easily regenerated using electrolytic regenerating techniques. However, they have such low etch rates, compared with the cupric chloride etchants, that they are not commercially feasible. The present invention is directed to improving dramatically the etch rate of these baths.
Various means of increasing the etch rate of copper etchants have been reported in the prior art. Illustratively, Saubestre U.S. Pat. No. 2,982,625 that the etch rate of a chromic acid-sulfuric acid bath containing a peroxysulfate is increased by the addition thereto of small amounts (0.05 to 5 g. per liter) of silver nitrate. Etching times for 2.8 mils thickness of copper were reported to be of the order of 17 minutes, i.e. 0.16 mil/minute.
Dutkewych et al. U.S. Pat. No. 4,144,119 describes the use of a combination of hydrogen peroxide and a molybdenum compound as a rate enhancer for a sulfuric acid etchant bath. Allan et al. U.S. Pat. No. 4,158,593 teaches the use of a catalytic amount of a selenium compound (selenium dioxide) and a secondary or tertiary alcohol to increase the etching rate and performance of a sulfuric acid-hydrogen peroxide bath.
Sykes U.S. Pat. No. 4,311,551 deals with alkaline ammoniacal copper etching baths and teaches the use as accelerating additives of cyanamide and precursors thereof including thiourea and dithiobiurea. The copper is present in the bath as a cupric salt which can include the chloride, nitrate, acetate, carbonate and ammonium sulfate. However the chloride is the salt employed in all the Examples and the etchant baths of the latter are all subject to the disadvantages described above.
Furst et al. U.S. Pat. No. 4,564,428 is concerned with alkaline ammonium sulfate copper etchant baths and describes the use of small amounts (0.05-0.4% w/w of chloride ion) of ammonium chloride to the regeneration time of the bath. Regeneration is achieved by bubbling oxygen through the bath. The amount of chloride ion introduced in this manner is said not to be a problem as far as generation of chlorine upon electrolytic recovery of the copper. A companion case, U.S. Pat. No. 4,557,811, describes the regeneration and recycling of the etchant baths of the '428 patent.